Wheelchair lifts of the type installed in the stairwell of transit vehicles, such as intracity buses, are well known. One type of wheelchair lift, commonly referred to as a "step lift," is illustrated in U.S. Pat. No. 4,466,771 to Thorley et at. (the "'771 patent"). The step lift is designed to be installed in the stairwell of a transit vehicle, and includes hinged panels that are movable between a step position and a platform position. In the step position, the hinged panels form steps for use by passengers to board and exit the vehicle. In the platform position, the hinged panels form a horizontal platform for use to raise and lower a wheelchair passenger between a vehicle floor-level position and a ground-level position. The hinged panels are attached to a carriage structure that may be driven upwards or downwards to raise or lower the platform.
Step lifts typically comprise a ramp/barrier plate which is pivotally mounted to the platform of the lift so as to be rotatable through an arc of about 180 degrees between a retracted position in which the ramp/barrier plate confronts and extends parallel to the platform and an extended position in which the ramp/barrier plate projects outwardly from the platform and is substantially coplanar with the platform. For ease of discussion, the ramp/barrier plate hereinafter will be referred to simply as the "barrier plate." The barrier plate provides a planar surface between the ground and the platform when the platform is in the lowered position, and acts as a vertically extending safety barrier (when in an intermediate position midway between the retracted and extended positions) for ensuring that a wheelchair does not roll off the platform when the platform is being moved between lower and upper positions.
Lifts of the type disclosed in the '771 patent typically include a linkage assembly for transmitting force from a hydraulic actuator to the barrier plate so as to cause the barrier plate to move between the retracted and extended positions. One portion of the linkage assembly is pivotally attached to the barrier plate, another portion of the linkage assembly is pivotally attached to the platform, and a third portion of the linkage assembly is attached to the hydraulic actuator. Generally, the linkage assembly is positioned in the center of the barrier plate and platform, i.e., midway between the left and fight sides of the barrier plate and platform as viewed from the roadside looking into the stairwell in which the lift is positioned.
Linkage assemblies of the type used in the '771 patent project above the surface of the barrier plate and platform more than is desired, e.g., as much as one inch. As a consequence of the central placement of the linkage assembly and its projecting configuration, a hump is formed which tends to interfere with a wheelchair occupant's use of the lift. Governmental regulations regarding the design of wheelchair lifts now prohibit projections that extend above the upper surface of the barrier plate and platform more than 0.25 inch. Clearly, linkage assemblies of lifts of the type disclosed in the '771 patent do not comply with this regulation.
Furthermore, the design of linkage assemblies of the type used on the lift of the '771 patent typically include undesirably large openings or gaps between the various elements making up the linkage assembly and between the linkage assembly and the apertures in the barrier plate and platform in which the linkage assembly is received. These gaps open out to the upper surfaces of the platform and the barrier plate and are sufficiently large that the heel of a high-heeled shoe, an end of a cane, or a child's foot could possibly become lodged within a gap between linkage elements. To avoid such an occurrence, flexible covers have been used to block gaps between elements of linkage assemblies. Such covers add to the cost of the linkage assembly, can adversely affect the operation of the linkage assembly, and tend to require frequent maintenance. Absent the use of such covers, the gaps between elements of known linkage assemblies do not comply with current governmental regulations which permit gaps up to only 0.625 inch wide.
The construction of linkage assemblies of the type used in the '771 lift is such that the various components thereof are formed by various machining operations. These operations tend to be relatively time consuming, and hence expensive. Consequently, the total cost of a linkage assembly of the type disclosed in the '771 patent is typically more expensive than is desired.
Linkage assemblies of the type disclosed in the '771 patent are generally designed so that access to the attachment point of the end of the linkage assembly coupled to the barrier plate is via the bottom surface of the barrier plate. Because such bottom surface is typically covered with a tread that covers the attachment point, and the process for removing and reinstalling the tread is relatively time consuming, removal of the linkage assembly for maintenance or replacement tends to be more difficult and time consuming than is desired.
The design of linkage assemblies of the type disclosed in the '771 patent is such that the barrier plate is driven downwardly toward the extended position with a relatively large force. This force is sufficiently great that if a person's feet are positioned underneath the barrier plate, the possibility exists that the person's feet could be crushed. To avoid the possibility of such an accident, a relief valve for limiting the pressure of the hydraulic fluid supplied to the actuator is provided. Such a relief valve adds to the cost of the lift.
Yet another problem with linkage assemblies of the type disclosed in the '771 patent is that the various components thereof require frequent lubrication, thereby adding to the cost of maintaining the linkage assembly. Furthermore, the design of the components of such known linkage assemblies is such that the components tend to corrode, thereby adversely affecting the free operation of the portions of the lift coupled to the linkage assembly and increasing the stresses applied to various components of the linkage assembly and the lift.
Furthermore, in the lift disclosed in the '771 patent, the actuator for movement of the barrier plate comprises a hydraulic cylinder mounted to the bottom of the curbside riser panel such that when the hinged panels are in the step position, the hydraulic cylinder for the barrier plate (hereinafter, "barrier cylinder") is in a substantially vertical position, as shown in FIG. 2 of the '771 patent. As the hinged panels move from the step position to the platform position, the barrier cylinder and the linkage assembly cause the barrier plate to move from the retracted position to the barrier position. If movement of the hinged panels or the barrier plate is hindered, for example, due to binding, or to striking an object, bending forces are applied to the barrier cylinder rod. The cylinder rod is designed only for the application of push/pull forces along the axis of the rod. If bending forces are applied to the cylinder rod, the barrier cylinder may be damaged.
It is also desirable for the step/platform assembly of a lift of the type disclosed in the '771 patent to be latched in the stowed position so that horizontal movement of the step/platform assembly is prevented in the event of failure of the platform extension cylinders. It is known to provide a step lift with a platform-mounted catch that engages a latch mounted at the rear of the lift. When the step/platform assembly is moved to the stowed position, the catch engages the latch such that the step/platform assembly is secured. In order to release the latch, it is required to actuate a cylinder to disengage the catch. This type of stow latch arrangement has a relatively complex construction and requires the provision of a dedicated actuating cylinder for the catch.
Consequently, a need exists for a linkage assembly for a step lift of the type disclosed in the '771 patent that does not project more than 0.25 inch above the upper surfaces of the platform and barrier plate and that does not include gaps of more than 0.625 inch. A need also exists for such a linkage assembly that can be serviced without removing the tread on the bottom surface of the barrier plate and that does not require periodic lubrication. A need further exists for a linkage assembly that is designed to drive the barrier plate downwardly toward the extended position with a force such that a person's feet inadvertently positioned in the path of travel of the barrier plate will not be crushed. A need additionally exists for a linkage assembly that is made from pans which can be fabricated quickly and inexpensively, preferably without the need for extensive machining operations. Moreover, a need exists for a barrier cylinder configuration that does not potentially subject the cylinder rod to bending forces. Furthermore, a latch mechanism for securing the step/platform assembly in the stowed position that has simple construction and does not require additional actuators or linkages is desirable.